1. Field of the Invention
The present invention relates to an optical pick-up apparatus for picking-up information out of an optical record medium by projecting a light beam upon the optical record medium and receiving a return light beam reflected by the optical record medium.
2. Description of the Related Art
There has been proposed an optical pick-up apparatus for picking-up information from a magneto-optical record medium. In general, such an information picking-up apparatus comprises a semiconductor laser emitting a linearly polarized laser beam, an objective lens for projecting the laser beam onto the magneto-optical record medium as a fine spot, an optical means for dividing a return laser beam reflected by the magneto-optical record medium into two light beams polarized in mutually orthogonal directions and a photodetecting means for receiving the two light beams. By suitably processing output signals supplied from the photodetecting means, it is possible to derive a reproduction signal representing information stored in the magneto-optical record medium, and to derive focusing error signal and tracking error signal representing a relative positional shift between the objective lens and the magneto-optical record medium.
In such a known optical pick-up apparatus, said optical means for dividing the return laser beam into the two light beams comprises a uniaxial birefringent crystal or double refraction crystal. For instance, Japanese Patent Application Publication Kokai Hei 2-37536 has proposed a known optical pick-up apparatus comprising a plane parallel plate made of birefringent material as shown in FIG. 1. As illustrated in FIG. 1, a light beam emitted by a light source 1 is divided into three beams, i.e. a single main beam and two sub-beams by means of a diffraction grating 2. These three light beams are reflected by a beams splitter 5 consisting of a half mirror 3 and a plane birefringent plate 4, and then are made incident upon an information record medium 8 by means of a collimator lens 6 and an objective lens 7. Three return beams reflected by the record medium 8 are made incident upon the beam splitter 5 by means of the objective lens 7 and collimator lens 6. Each of the three return beams is divided by the beam splitter 5 into first and second light beams which are then made incident upon photodetectors 9 and 10.
In the known optical head illustrated in FIG. 1, each of the three return beams is given with astigmatism and is divided into the first and second light beams. The first light beams of the three return beams are received by the photodetector 9 separately from one another, and the second light beam of the main beam is received by the photodetector 10. By processing the output signals from the photodetectors 9 and 10, there are produced a focusing error correction signal and a tracking error correction signal. In this prior art publication, there is not explained how to derive the reproduced information signal, but judging from an explanation with reference to FIG. 6 of the publication, it is presumed that the reproduction signal is derived from a difference between a sum of output signals from four light receiving regions of the photodetector 9 receiving the first light beam of the main beam and the output signal of the photodetector 10 receiving the second light beam of the main beam.
Japanese Patent Application Publication Kokai Sho 64-27055 has proposed another known optical pick-up apparatus comprising a wedge-shaped prism made of anisotropic crystal. As shown in FIG. 2, a laser beam emitted by a semiconductor laser 11 is converted into a parallel light beam by means of a collimator lens 12 and then is made incident upon an optical record medium 16 by means of first and second beam splitters 13 and 14 and objective lens 15. A return laser beam reflected by the record medium 16 is made incident upon the second beam splitter 14 by means of the objective lens 15, and a light flux reflected by the second beam splitter 14 is made incident upon a wedge-shaped prism 18 by means of a converging lens 17. Then, the light flux is divided into a P polarized component and an S polarized component which are separately made incident upon two light receiving regions of a photodetector 19. A part of the return beam transmitted through the second beam splitter 14 and reflected by the first beam splitter 13 is made incident upon an error detecting optical system 20.
The wedge-shaped prism 18 is made of anisotropic crystal and has incident and exit surfaces which are not parallel with each other. When the return laser beam is made incident upon the prism 18 by means of the converging lens 17, the return laser beam is divided into two beams which are subjected to astigmatism. These two beams are received by the photodetector 19 having two light receiving regions and arranged in a vicinity of a plane on which diameters of the two beams viewed in a refracting direction become minimum. It should be noted that in FIG. 2, for the sake of simplicity, the photodetector 19 is arranged behind a plane A, but in practice the photodetector is provided on the plane A. By deriving a difference between output signals from the two light receiving regions, it is possible to obtain the reproduced information signal.
In Japanese Patent Application Laid-open Publication Kokai Sho 63-161,541, there is proposed another known optical head using a composite prism element as shown in FIG. 3. In this known optical head, a laser beam emitted by a semiconductor laser 21 is made incident upon a composite prism element 23 by means of a collimator lens 22 and a light beam reflected by the composite prism element is made incident upon a magneto-optical record disk 25 by means of an objective lens 24. A return laser beam reflected by the record disk 25 is made incident upon the composite prism element 23 via the objective lens 24 and is divided into a first beam L1 composed of a P polarized component and a second beam L2 composed of an S polarized component whose polarizing direction is perpendicular to that of the P polarized component. These two light beams are made incident upon a photodetector unit 27 by means of a collimator lens 26. Output signals from the photodetector unit 27 are supplied to a signal processing unit 28 to derive reproduced information signal S.sub.i, focusing error signal S.sub.f and tracking error signal S.sub.t. The composite prism element 23 comprises a glass prism 29 and rock crystal or quartz prism 30 which are cemented to each other by means of a multi-coating film 31 and adhesive agent layer 32 and has a hexogonal shape.
In preliminary documents No. 3, 28a-SF-19 and 28a-SF-20 for 54th Applied Physics Conference, there disclosed "Hologram Laser Unit for CD optical pick-up".
In a known optical pick up disclosed in the preliminary document No. 3, 28a-SF-19, in a silicon substrate surface in which a photodetector (PD) is formed there is provided by etching a recess having a pyramid flastum shape and a semiconductor laser chip (LD) is arranged on a bottom surface of the recess, and the silicon substrate is provided in a package. Above this package is arranged a hologram optical element (HOE) having a grating pattern formed in a lower surface and a hologram pattern formed in an upper surface. A laser beam emitted by LD is divided into three beams by means of the grating pattern and these three beams are made incident upon an optical disk, and three return beams are subjected to the diffraction and wave surface transformation by the hologram pattern. Laser beams emanated from the hologram pattern are received by PD to detect RF signal and servo signals.
In the preliminary document No. 3, 28a-SF20, there is disclosed that a focus error signal is derived from output signals of PD in accordance with the beam size method and a tracking error signal is derived by the three-beam method.
In the known optical pick-up illustrated in FIG. 1, spot diagrams of the first and second beams of the main beam on the photodetectors 39 and 40 were calculated under the following condition: the bire-fringent plate 4 is formed by LiNbO.sub.3 crystal plate having a thickness of 1.5 mm and the return beam from the information record medium 8 has a numerical aperture of 0.15. A result of this calculation is shown in FIG. 4. As can be seen from the diagram shown in FIG. 4, the spots 35a and 35b of the first and second beams impinging upon the photodetectors are not separated from each other, but are mutually overlapped. This is due to the fact that the optical axes of the first and second beams are separated in parallel with each other within the birefringent plate 4, so that a separation distance is small.
It is considered to increase the separation distance by increasing a thickness of the birefringent plate 4. Then, astigmatism and coma are increased and a size of the spots 35a and 35b becomes larger. This results in that the overlap of the spots could not be mitigated effectively.
In the known optical pick-up depicted in FIG. 2, use is made of the wedge-shape prism 18 made of anisotropic crystal, and thus exit angles of the two orthogonally polarized components emanating from the prism can be differed from each other. For instance, it has been described that the wedge-shape prism 18 is made of rutile and is arranged such that the return beam from the record medium is made incident upon the prism perpendicularly and a direction of the optical axis is set in parallel with the P polarized beam or the S polarized beam. Then, the P polarized beam and S polarized beam emanate from the prism in different directions having an angle of about 3.degree..
In this manner, the method of separating the beams by providing the angles between these beams is superior to the parallel separation method shown in FIG. 1. This has been confirmed by a following calculation. Now it is assumed that the wedge prism 18 is made of rutile having a thickness of 1.5 mm at a portion through which an optical axis passes, the return beam is made incident upon the prism perpendicularly and the exit surface is inclined by 10.degree. with respect to the incident surface, and the crystal axis is set to 45.degree. with respect to the P and S polarizing directions. Then, spot diagrams were calculated by using a thin lens having a focal length of 20 mm and a numerical aperture of 0.15 as the converging lens 17 arranged in front of the prism. A result of this calculation is represented in FIGS. 5A and 5B. FIG. 5A illustrates a spot diagram at the position A in FIG. 2 at which a diameter of the beam viewed in a direction of refraction becomes minimum, and FIG. 5B depicts a spot diagram at the position B in FIG. 2 at which a cross section of the beam becomes substantially circular.
In the publication disclosing the known optical pick-up apparatus shown in FIG. 2, there is explained that the photodetector 19 including the two light receiving regions has to be placed in a vicinity of the position at which a size of the beam viewed in the direction of refraction becomes minimum in order to receive the two return beams separately from each other. However, judging from the spot diagram illustrated in FIGS. 5A and 5B, it is not necessary to provide the photodetector at such a position. This is due to the fact that the beam is not sufficiently confined into a focus line at the position at which the beam size becomes minimum, and the P polarized component and S polarized component can be separately received from each other when these components emanate from the wedge-shaped prism 18 into different directions forming an angle of about 3.degree. therebetween.
The known optical pick-up shown in FIG. 2 has drawbacks that there have to be provided two beam splitters, the beam splitter 14 for introducing the return beam from the record medium 16 into the optical system including the wedge-shaped prism 18 for detecting the rotation angle of the polarizing direction, and the beam splitter 13 for introducing the return beam transmitted through the beam splitter 14 into the error detecting optical system 20. This apparently results in an increase in cost and size of the optical pick-up apparatus.
In the known optical pick-up apparatus depicted in FIG. 3, since the composite prism element 23 is used, the beam splitter and analyzer may be eliminated. Therefore, the number of components of the optical pick-up can be reduced. Furthermore, in the optical pick-up apparatus shown in FIG. 3, by making the return beam from the magneto-optical disk 25 incident upon the composite prism element 23 via the objective lens 24, the first P polarized component and second S polarized beam are separated such that an optical axis of the P polarized beam makes an angle of .theta.e in the P polarization plane of the return beam with respect to a normal to a boundary surface between the glass prism 29 and the quartz prism 30 constituting the composite prism element 23 and an optical axis of the S polarized beam makes angle of .theta.o in the P polarization plan of the return beam with respect to said normal to the boundary surface between the prisms 29 and 30 (here .theta.e&gt;.theta.o).
The light beam incident upon the objective lens 24 has been converted into the parallel beam by means of the collimator lens 22, and thus the return beam impinging upon the composite prism element 23 is also a parallel beam. Therefore, the first P polarized beam and S polarized beam emanating from the composite prism element 23 have no aberration, so that the beams impinging upon the photodetector unit 27 via the common lens 26 are converged as very fine spots which are sufficiently separated from each other on the photodetector unit.
However, in the known optical pick-up apparatus illustrated in FIG. 3, the first and second return beams separated by the composite prism element 23 are made incident upon the photodetector unit 27 by means of the common converging lens 26, and therefore it is necessary to form a space which corresponds to a focal length of the converging lens and the whole pick-up apparatus is liable to be large in size. Moreover, in the photodetector unit 27, a center of a group of light receiving regions receiving the first return beam has to be coincided with the optical axis of the first return beam and a center of a group of light receiving regions receiving the second return beam has to be aligned with the optical axis of the second return beam, so that a rather strict adjustment of positions of various parts is required and this results in an increase in cost.
Furthermore, in the publication disclosing the optical pick-up apparatus shown in FIG. 3, there is explained that shapes of spots of the first and second return beams on the photodetector unit are detected and a focusing error signal is derived on the basis of detected beam spot shape. However, there is not explained at all in what way the beam spot shape of the first and second return beams changes.
In the optical pick-up apparatus shown in FIG. 3, since the laser beam emitted by the semiconductor laser 21 is converted into the parallel beam by means of the collimator lens 22 and then is made incident upon the magneto-optical disk 25 by means of the objective lens 24, when the objective lens becomes in defocus condition, the return beam impinging upon the composite prism element 23 is no more parallel. Therefore, when the return beam is transmitted through the composite prism element 23, there is introduced aberration. This aberration becomes manifest when a difference in refractive index between the glass prism 29 and the quartz prism 30 of the composite prism element 23 is large even if a degree of defocus is small. Then, the beam spots of the first and second return beams on the photodetector unit become too large to be detected separately from each other.
In the "Hologram Laser Unit for CD optical pick-up" disclosed in the above mentioned preliminary documents No. 3, 28a-SF-19 and 28a-SF-20 for 54th Applied Physics Conference, there are not provided a means for dividing the return beam from the record medium into the two mutually orthogonally polarized beams and a means for detecting a change in intensity of these divided beams, and thus this known apparatus could not be applied to the magneto-optical record medium.
FIG. 6 is a perspective view showing another known optical pick-up apparatus described in Japanese Patent Application Publication Kokai Hei JP 1-315721. In this known optical pick-up apparatus, a laser beam 42 emitted by a semiconductor laser 41 is converted by a collimator lens 43 into a parallel beam 44 and is made incident upon a beam splitter 45. The laser beam reflected by a reflection surface 45a of the beam splitter 45 is further reflected by a mirror 46 and is made incident upon a magneto-optical record disk 48 by means of an objective lens 47. A laser beam 49 reflected by the magneto-optical disk 48 is converted into a parallel beam 50 by means of the objective lens 47 and is then reflected by the mirror 46 toward the beam splitter 45. A parallel laser beam 51 transmitted through the reflection surface 45a is converged by means of a detection lens 52 and a concave lens 53 into a converging beam 54. Then, the converging beam 54 is made incident upon a polarizing beam splitter 55 and is divided thereby into P and S polarized components, and these components are received by a photodetector 56.
FIG. 7 is a schematic view showing the construction and operation of the polarizing beam splitter 55. The polarizing beam splitter 55 comprises a parallelogram prism 55b, a first transparent plane parallel plate 55c provided on one surface of the prism 55b via a polarizing film 55a which transmits the P polarized component and reflects the S polarized component, said plane parallel plate being made of an optical material having the same refractive index as that of the prism 55b and having a total reflection surface 55d. The polarizing beam splitter 55 further comprises a second transparent plane parallel plate 55f secured to other surface of the prism 55b via an anti-reflection film 55e and having a total reflection surface 55g, said second plane parallel plate being made of an optical material having a refractive index different from that of the parallelogram prism 55b. The polarizing beam splitter 55 is arranged such that an incident plane for the incident laser beam 54 is rotated by 45 degrees with respect to the polarizing direction of the laser beam emitted by the semiconductor laser 41.
In this known optical pick-up apparatus, the laser beam 54 impinging upon the polarizing beam splitter 55 is divided by the polarizing film 55a into a P polarized component transmitted through the polarizing film and an S polarized component reflected by the polarizing film. The P polarized beam transmitted through the polarizing film 55a is reflected by the total reflection surface 55d of the plane parallel plate 55c and is transmitted through again the polarizing film 55a. In this manner, the P and S polarized components are made incident upon the plane parallel plate 55f made of a material having different refractive index from that of the parallelogram prims 55b and are reflected by the total reflection surface 55g. In this manner, the P and S polarized beams 54a and 54b each having astigmatism emanate from the polarizing beam splitter 55 separately from each other.
The P and S polarized beams 54a and 54b are detected by the photodetector 56 and outputs of the photodetector are suitably processed to derive a focusing error signal in accordance with the astigmatism method as well as a reproduced RF signal by deriving a difference in intensity between the P and S polarized beams.
In the known optical pick-up apparatus shown in FIG. 6, the polarizing beam splitter 55 also serves to give the divided P and S polarized beams 54a and 54b the astigmatism, and thus the number of elements can be reduced. However, in order to detect the information from the magneto-optical disk 48, it is necessary to arrange the polarizing beam splitter 55 to be rotated by 45 degrees with respect to the polarizing plane of the laser beam 42 emitted by the semiconductor laser 41. This results in that the adjustment or assembly becomes complicated. Further, since the incident beam path and the return beam path are separated by the beam splitter 55, the optical elements of the incident path side and the optical elements of the return beam side are arranged at right angles and thus the whole size of the pick-up apparatus is liable to be large.
FIGS. 8 and 9 illustrate another known optical pick-up apparatus for reading the information out of the magneto-optical information record medium described in Japanese Patent Application Publication Kokai Hei 5-120755. In this known pick-up apparatus, a laser beam produced by a semiconductor laser 62 mounted on a silicon substrate 61 is reflected upwardly by a mirror 64 provided on the silicon substrate. The laser beam is then made incident upon a magneto-optical record medium 67 by means of a hologram element 65 and an objective lens 66. The hologram element 65 includes a first hologram 65a provided on a surface of the hologram element which faces with the silicon substrate 61, said first hologram having gratings extending in a direction substantially parallel with a direction X of an information track on the magneto-optical record medium 67 and further having a lens function for giving opposite refractive powers to .+-.1-order diffracted beams, and a second hologram 65b provided on a surface opposite to said surface 65a and having gratings which extend substantially in parallel with a direction Y which is perpendicular to the track direction X. 0-order beams transmitted through these first and second holograms 65a and 65b are made incident upon the magneto-optical record medium 67 by means of the objective lens 66.
The return beam reflected by the magneto-optical record medium 67 is made incident by the objective lens 66 upon the second hologram 65b of the hologram element 65 and .+-.1 order beams diffracted thereby are received by third and fourth photodetectors 63C and 63D formed on the silicon substrate 61 via first and second polarizing beam splitting elements 68A and 68B including first and second fine gratings 68a and 68b, respectively. The first and second polarizing beam splitting elements 68A and 68B are provided on the third and fourth photodetectors 63C and 63D, respectively, and the first and second fine gratings 68a and 68b are formed to be inclined by .+-.45 degrees with respect to the direction Y, so that they degrees with respect to the direction Y, so that they cross with each other at right angles. Each of the third and fourth photodetectors 63C and 63D includes two light receiving regions 63g, 63h and 63i, 63j which are bisected along a line extending in the direction X.
In this manner, the 0-order beams transmitted through the first and second fine gratings 68a and 68b are received by the third and fourth photodetectors 63C and 63D, and output signals of these photodetectors are processed to derive the RF reproduced information signal and the tracking error signal by the push-pull method.
The 0-order return beam transmitted through the second hologram 65b is made incident upon the first hologram 65a and is divided thereby into .+-.1-order diffracted beams having opposite focal powers. Then, these .+-.1 order diffracted beams are received by the first and second photodetectors 63A and 63B separately from each other. Here, each of the first and second photodetectors 63A and 63B includes three light receiving regions 63a, 63b, 63c, 63d, 63e, 63f divided along lines extending in the direction Y. In this manner, by processing output signals from the first and second photodetectors 63A and 63B, it is possible to derive the focusing error signal in accordance with the beam size method.
In Japanese Patent Application Publication Kokai Hei 3-212828, there is disclosed still another known optical pick-up apparatus. As shown in FIG. 10, various optical elements such as semiconductor laser 71, trapezoidal prism 72 and photodetectors 73a, 73b are arranged within a package 74. The trapezoidal prism 72 is made of a birefringent material and its upper surface 72a is inclined by 45 degrees with respect to an optical axis and a half mirror 75 is provided on this surface. A laser beam emitted by the semiconductor laser 71 is made incident upon the half mirror 75 and a laser beam reflected by the half mirror is transmitted through a transparent glass window 76 formed in the package 74. The laser beam is then made incident upon a magneto-optical record medium 78 by means of an objective lens 77. A return laser beam reflected by the magneto-optical record medium 78 is converged by the objective lens 77, is transmitted through the glass window 76 and is made incident upon the half mirror 75. A return beam transmitted through the half mirror 75 is made incident upon the trapazoidal prism 72 and emanates from the prism from its lower surface 72b, thereby being subjected to astigmatism as well as being divided into two orthogonally polarized beams which are then received by the photodetectors 73a and 73b each having four divided light receiving regions.
It is possible to derive a focusing error signal by processing output signals from the first and second photodetectors 73a and 73b in accordance with the astigmatism method and a RF reproduced signal can be obtained by deriving a total sum of the output signals of the four light receiving regions of the second photodetector 73b.
In the known optical pick-up apparatus illustrated in FIG. 8, the focusing error signal and the RF information signal are derived by detecting the laser beam diffracted by the first hologram 65a and the laser beam diffracted by the second hologram 65b separately from each other, and thus the detection of the focusing error signal is not influenced by the first and second polarizing beam splitting elements 68A and 68B and the focusing error signal can be obtained precisely and accurately. Moreover, since the light receiving region dividing lines of the photodetectors 63A and 63B are in parallel with the diffracting direction of the corresponding gratings, a movement of the spots on the photodetectors due to change in a wavelength of the laser beam may be in parallel with the dividing lines. Therefore, even if the wavelength of the laser beam fluctuates, there is not produced an off-set in the tracking error signal and focusing error signal.
However, the inventors have found that this known optical pick-up apparatus has still the following disadvantages.
In general, in order to record information on the magneto-optical record medium and to erase the information recorded on the magneto-optical record medium, it is necessary to irradiate the record medium with the light spot having a high intensity. To this end, it is necessary to increase the transmissivity of the first and second holograms 65a and 65b for the 0-order beam (for instance not less than 70%), so that the laser beam from the semiconductor laser 62 can be focused on the magneto-optical record medium 67 through the hologram element 65 and objective lens 66 in an efficient manner.
However, if the transmissivity of the hologram element 65 for the 0-order beam is increased, the diffracting efficiency for the .+-.1-order beams becomes low (not higher than 70%), an amount of the .+-.1-order return beams diffracted by the second hologram 65b becomes very small, and almost all the return beam containing the signal component is transmitted through the second hologram 65b as the 0-order beam. Therefore, a loss of the signal component is too small to detect the RF information signal from the magneto-optical record medium at a high C/N.
In the above mentioned Japanese Patent Application Publication Kokai Hei 3-212828 disclosing the known optical pick-up apparatus illustrated in FIG. 10, page 4, left lower column, there is described that a detectable range .delta. of the focusing error may be expressed by the following equation: ##EQU1## wherein L is a distance between the upper and lower surfaces 72a and 72b of the trapezoidal prism 72, n is a refractive index of the prism 72 and M is a lateral magnification of the objective lens 77. In the same publication, page 4, right upper column, there is shown an example, in which M=1/5, L=2.0-3.0 mm, and a detectable range of the focusing error becomes 10-15 .mu.m which is comparable to currently used optical pick-up apparatuses.
There has been no consideration for a case in which the trapezoidal prism 72 is made of birefringent material, but it is now assumed that the trapezoidal prism is made of quartz. A refractive index of the quartz for ordinary light is about 1.539 and that for extraordinary light is about 1.548. As explained above, when the magneto-optical record medium is used, the laser beam emitted by the semiconductor laser 71 has to be focused onto the record medium at a relatively high efficiency, so that the lateral magnification M of the objective lens 77 has to be about 0.273. By considering the above matters, the equation (1) is solved for L by setting .delta.=10 .mu.m, n=1.539 and M=0.273. Then, L=1.05 mm is obtained. In the equation (1), it is questionable why the numerical aperture of the objective lens 77 is not contained as a parameter. Now a spot diagram on the photodetectors 73a and 73b is calculated by using the above values. Then, a spot diagram shown in FIG. 11 can be obtained. As depicted in FIG. 11, on the photodetectors, the ordinary light spot and extraordinary light spot overlap each other and thus they could not be detected separately.
In the above publication, there is also shown a trapezoidal prism 72 consisting of two triangle or trapezoidal prisms 72-1 and 72-2 made of different birefringent materials as shown in FIG. 12. Then, the P and S polarized beams are separated from each other by a larger angle so that they may be detected separately from each other by the photodetectors 73a and 73b. In this case, if the focusing error is to be detected by the astigmatism method which is most common in optical pick-up apparatuses for optical information reproducing apparatuses, the material of the prisms and a value of L have to be set to obtain a given astigmatism, and thus the P and S polarized return beams could not be detected separately from each other.
Therefore, even if a difference between a sum of output signals of the four light receiving regions of the first photodetector 73a and a sum of output signals of the four light receiving regions of the second photodetector 73b is derived, it is impossible or difficult to derive the information signal from the magneto-optical record medium 78.
Moreover, in the known optical pick-up apparatus illustrated in FIG. 10, the return laser beam reflected by the magneto-optical record medium 78 is made incident upon the trapezoidal prism 72 by means of the half mirror 75, and therefore a half of the return beam is lost by the half mirror.